The invention relates to a coating device for coating a size-press roll, paper or board or an equivalent moving base. The coating device comprises a revolving coating bar which rests against the moving base and extends across the machine width. The coating bar is supported in a cradle substantially over its entire length and is fitted to spread and to smooth a coating agent onto the moving base. The coating agent is introduced into the coating device in the direction of running of the moving base before the coating bar.
The invention is further related to a coating device for coating a size-press roll, paper or board or an equivalent moving base, comprising a coating-agent chamber including a revolving coating bar supported against a moving base and which acts as a coating member. The coating bar extends across the machine width, and the coating-agent chamber is defined by the front wall of the coating-agent chamber, by the lateral seals of the coating device, and by the moving base. The coating agent is arranged to be fed into the coating-agent chamber under pressure.
At present, in the coating of paper or board, two alternative methods and devices are commonly used, i.e. a blade coater or a bar coater. The present invention is expressly related to the latter, bar coaters, which have proved excellent especially in the film size press technique. The material of the coating bars currently in use is usually steel, and, in view of increasing the service life of the bar, the bars are provided with chromium plating. In surface sizing of paper, bars with fully smooth faces have not been used. Rather, the face of the coating bar has been provided with grooves, or alternatively steel wire is wound onto the bar to form an arrangement similar to grooves on the bar face. A grooved bar is used for the surface sizing of paper because the thickness of the size film to be applied onto the base to be coated is determined by means of the depth of the grooves.
A grooved bar is, however, poorly suitable for the preparation of thin size films, because it is very difficult to manufacture grooves of sufficiently small scale. On the other hand, such small grooves are easily contaminated. It is a further highly significant drawback of grooved bars that they are worn rapidly. Pigmenting with a high dry solids content is also entirely impossible with grooved bars, because the wear of the bars is excessive in this connection. The diameters of the coating bars currently in use have been very small. Small-diameter bars are used because the bar ought to be able to adapt itself to the shape of the roll face in the size press in order that the profile of the size film could be made suitable and correct. Generally, the order of magnitude of the diameter of currently used coating bar is about 10 mm.
It has not been possible to use bars of large diameter because, owing to their thickness, they have been too rigid to provide an adequate profiling. In respect to the prior art, reference is made in particular to U.S. Pat. No. 2,970,564, from Which a device is known for spreading of a size film or of a pigment coating film onto the rolls in a size press, said films being transferred to the paper in the nip in the film size press. In the method described therein, smooth or grooved bars are used for the metering of the size film. It is stated therein that while it is possible to use a smooth-faced bar of small diameter (diameter about 9 mm) for surface sizing, such a bar is unsatisfactory. Accordingly, a corresponding device has never been applied in practice. In respect of the prior art, reference is further made to FI Patent No. 30,147.